1. Field of the Invention
The present invention relates to a communications system, and more particularly to a voice mail service system for a private switching system.
2. Background of the Related Art
A private switching system is an appliance for switching the connection of communication channels between a central office line and an extension, or between extensions. The private switching system typically provides a voice mail service. The voice mail service includes an automatic attendant service to guide a state of an extension subscriber to a caller-side terminal when an incoming call is attempted. It does this by means of a voice message when there is no response to the call due to an absence of the extension subscriber or other reason, or when the called extension subscriber's line is in an off-hook state. In order to provide such a voice mail service for a caller who has effected an incoming call to a certain extension subscriber, a separate voice mail service system capable of providing the voice mail service has to be installed in the private switching system.
As shown in FIG. 1, a related art private switching system 1 includes a network connection section 1a connected to a Public Switched Telephone Network (PSTN) 4, a subscriber section 1b having information about subscribers, and a switching section 1c for connecting the network connection section 1a and the subscriber section 1b with each other.
A voice mail service system 2 includes a subscriber matching section 2b interfacing with the subscriber section 1c of the private switching system 1. It further includes a voice signal processing section 2a for compressing service voice signals inputted in a pulse code modulated (CM) state. The voice signal processing section 2a also restores the service voice signal having been compressed in a prescribed mode, and transmits the restored service voice signal through the private switching system 1 to the caller's terminal when an incoming call is detected. By doing this, it can provide the voice mail service. The voice mail service system 2 also includes a voice data storage section 2c for storing data inputted through the voice signal processing section 2a. 
A control section 3 controls the operation of the entire system. It performs functions of exchanging call-related messages with the private switching system 1, which is a system of higher rank, exchanging voice messages for providing the voice mail service through the voice signal processing section 2a, and controlling the voice data storage section 2c so as to perform general operations including a recording and a reproduction of processed voice data.
The related art private switching system has various problems. For example, because the voice mail service system is a separate system, it requires a high additional expense. Accordingly, when a private switching system has a relatively small number of extensions, the switching system usually does not provide the voice mail service. Therefore, when an extension subscriber does not answer an incoming call, the caller, who has made the incoming call, is not informed of the state of the extension subscriber. Thus, the caller is apt to repeatedly apply the incoming call, thereby unnecessarily occupying the communication network and adding an extra load on the private switching system.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.